Tensions between Baghdad’s two most important partners could pose challenges once Islamic State is defeated

MOSUL, Iraq—In the battle against Islamic State, the U.S. and Iran have become de facto allies in Iraq, a convergence of interests that permitted both nations to tacitly cooperate and avoid open conflicts.

Once Islamic State is defeated, however, Washington’s and Tehran’s interests are likely to diverge, especially if President Donald Trump makes good on his pledge to aggressively contain Iranian power in the region.